In order to feed an infinitesimal quantity of a highly pressurized gas from the primary side of a gas passage to the secondary side thereof, there has been used a needle regulator which comprises a tapered needle axially movable relative to a cylindrical value passageway to regulate gas flow through the needle regulator. However, since the gap between the wall of the valve passageway and the needle should be as small as less than 1 micron, it has been difficult to center the needle relative to the wall of the valve passageway, and as a result, the gas flowing therethrough could not be precisely regulated. In addition, in case a highly pressurized gas flows through the narrow gap at a more or less sonic velocity, the needle is cooled due to the heat absorption by the gas produced by its adiabatic expansion, and therefore the needle is axially contracted. This causes the substantially same result as that accomplished by withdrawing the needle, and therefore, the degree of opening of the regulator tends to vary to a relatively large degree.